Pneumomediastinum is a rare complication of dermatomyositis (DM) and Polymiositis (PM). We report here three cases of PM/DM who developed pneumomediastinum. First case was 61 years old woman with amyopathic dermatomyositis (aDM). Her aDM was complicated with skin ulceration due to vasculopathy, but complicated interstitial pneumonia was not severe. She developed subcutaneous emphysema and pneumomediastinum. Second case was 57 years old woman with DM, who had intractable skin phenomena and mild interstitial pneumonia. The patient became subcutaneous emphysema and pneumomediastinum following severe vasculopathy of skin. The last case was 63 years old man with PM. His PM was complicated with interstitial pneumonia. He had intractable respiratory symptom. Ten years later, he became subcutaneous emphysema and pneumomediastinum following pneumothorax. First and second cases suggest that their pneumomediastinum were due to vasculopathy. On the other hand, pneumomediastinum of the last patient seemed to be associated with interstitial pneumonia and steroid.